1. Field of the Invention
This invention is generally directed to an ion mobility spectrometer and more particularly to a spectrometer for sampling the atmosphere for use as a portable device for detecting contaminants in the air.
2. Background of the Invention
Ion mobility spectrometry has been developing as a technique for detecting contaminants in gas samples for 25 to 30 years. Ion movement is dependent upon mobility and is therefore related to the size and charge of the ions. Primarily only singly charged ions are observed with an IMS. The IMS device consists of an atmospheric pressure ionizer coupled to a drift tube spectrometer. Sample molecules that are ionized by ion-molecule reactions enter a drift region where they are separated according to mobility. A radioactive source, usually Ni-63, ionizes a carrier gas that is introduced into reaction region to form reactant ions. In the presence of the vapor sample having the contaminant to be identified, the reactant ions undergo ion-molecule reactions with the sample molecules to form product ions. An electric field gradient present throughout the tube causes the product ions to be introduced into the drift region.
A shutter grid separating the reaction region from the drift region interrupts the flow of ions during this process. The shutter grid is an array of parallel wires that are alternately biased so that the application of equal potentials to alternate wires will permit the product ions to pass to the drift region, while unequal potentials will deflect the arriving ions and prevent their passage into the drift region. Typically, the shutter grid is pulsed every 40 milliseconds to transmit the ions into the drift region. Upon entering the drift region the ions travel toward a collector plate under the influence of the electric field gradient.
A drift gas is introduced into the drift region to quench any remaining reactions and to collide with the product ions to produce a drift velocity unique for each contaminant. The ions arriving at the collector plate are measured as current peaks separated by the arrival time. A detector analyzes the current peaks to provide a mobility signature to permit the identification of the presence of a particular contaminant.
The prior art ion mobility spectrometers (IMS) have major disadvantages for direct application in atmospheric sensing. The sample vapor introduced into the present IMS tubes must be considerably manipulated. In addition, the prior IMS tubes utilize a carrier gas that must be purified air or prepurified nitrogen. If ambient air is used as the carrier gas, the ammonia and water components of the air alter the identity of the reactant ions that may interfere with the IMS response to the sample. Thus, large pressured tanks are required to purify and introduce the carrier gas into the reaction region. Moreover, the drift gas must also be a purified gas that is introduced into the drift region from the pressure tanks. Provision is made to release the purified carrier and drift gases near the shutter grid.
One prior art IMS system which uses ambient air as the carrier gas is disclosed in Spangler et al. Developments In Ion Mobility Spectrometry, 23 ISA Transactions No. 1, 17-28 (1984). The Spangler et al. IMS tube utilizes a specially designed membrane inlet that allows a continuous recycled nitrogen stream with sample to be drawn across the external surface of the membrane while the carrier gas flows across the interior surface of the membrane. The membrane excludes such elements as ammonia, water and nitrogen oxide. However, this system still requires a means for purifying the carrier and drift gases, thus not overcoming the limitation of the prior art IMS tubes of being impractical for portable contaminant testing.